1. Field of the Invention
Embodiments relate to a method of controlling a memory cell of a non-volatile memory device.
2. Description of the Related Art
A non-volatile memory device can electrically erase and program data, and can retain the data even when not being supplied with power. A common example of the non-volatile memory device is a flash memory which stores data based on a charge storage. Each of memory cells of the flash memory is formed of a cell transistor including a control gate, a charge storage layer, a source, and a drain. The flash memory changes a data value written to the memory cell by adjusting the amount of charge of the charge storage layer. The cell transistor of the flash memory adjusts the amount of charge of the charge storage layer according to an Fowler-Nordheim (F-N) tunneling mechanism.
An erasing operation of the cell transistor is performed by applying a ground voltage to the control gate of the cell transistor, and by applying a high voltage, higher than a power voltage, to a semiconductor substrate (or, a bulk). According to such a bias condition for erasing, a strong electric field is formed between the charge storage layer and the bulk, due to a high voltage difference between the charge storage layer and the bulk. As a result, charges in the charge storage layer are emitted to the bulk due to an F-N tunneling effect. At this point, a threshold voltage of the erased cell transistor is lowered.
A programming operation of the cell transistor is performed by applying the high voltage, higher than the power voltage, to the control gate, and by applying the ground voltage to the drain and the bulk. According to such a bias condition for programming, the charges are injected into the charge storage layer of the cell transistor due to the F-N tunneling effect. Accordingly, the threshold voltage of the cell transistor is increased.
In an erase state, the threshold voltage of the cell transistor is a negative (<0) voltage, since charges exits the charge storage layer. In a program state, the threshold voltage of the cell transistor is a positive (>0) voltage, since charges are injected into the charge storage layer.